Resin composition for sealing electronic parts, and hydration-resistant magnesia powder and process for preparation thereof

ABSTRACT

Disclosed is a resin composition for sealing electronic parts, which comprises a thermosetting resin and, incorporated therein, high-purity magnesium oxide. A hydration-resistant magnesia powder having the particle surfaces covered with a continuous and uniform coating of silica is advantageously used as the magnesium oxide. This hydration-resistant magnesia powder is prepared by introducing a heated vapor of an organic silicate compound into a reactor of a fluidized bed of magnesia powder heated at 100° to 600° C. so that the concentration of the organic silicate compound is 1 to 20 mole %, and precipitating a coating of silica on the surfaces of magnesia particles by thermal decomposition and/or hydrolysis of the organic silicate compound on the surfaces of magnesia particles.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a resin composition for sealingelectronic parts, which is excellent in thermal conductivity and metalabrasion resistance. Furthermore, the present invention relates to amagnesia powder in which the hydration resistance is highly improved bycovering the surfaces of particles of the magnesia powder with a uniformcoating of silica, and a process for the preparation thereof.

2. Description of the Related Art

The method for resin-sealing electronic parts such as semiconductors,resistors, condensers and coils with thermosetting molding materials isvigorously adopted in these years. The requirement for the reliabilityof each electronic part is increasing and hence, improvements ofcharacteristics of sealants are eagerly desired. For example, the degreeof integration of semiconductors is increased and the quantity of heatgenerated is increased with increase of the integration degree, andtherefore, the heat dissipation of resin-sealed electronic parts becomesa problem. For solving this problem, use of a sealant having a highthermal conductivity is desired.

From the economical viewpoint and in view of heat resistance, moistureresistance and adhesion, in most of conventional sealants, an epoxyresin is used as the thermosetting resin and silica powder is added asan inorganic filler for improving thermal conductivity or mechanicalstrength. Most of available silica powders are composed of crystallinesilica or fused silica. Resin-sealing materials comprising these silicapowders, however, are still defective in that the thermal conductivityis not sufficient to seal a highly integrated electronic part having alarge quantity of generated heat. Furthermore, since the hardness ofsilica powder is high, a resin-sealing material comprising silica powderwears a molding machine or mold at the transfer molding or injectionmolding, and this sealing material is not suitable for the long-periodoperation. Therefore, development of an improved a resin-sealingmaterial is desired.

Magnesia is excellent in heat resistance, thermal conductivity andelectrically insulating property and a high-density sintered body ofmagnesia has a high transparency. By dint of these characteristics,utilization of magnesia powder as the material of fillers or magnesiaceramics has been examined. However, magnesia powder is practically usedonly as a material of heat-resistant magnesia ceramics such as acrucible or a protecting tube of a thermocouple. The reason is thatmagnesia is poor in the hydration resistance and especially in thepowdery state, magnesia is readily hydrated by water contained in airand converted to magnesium hydroxide, resulting in drastic degradationof characteristics. As means for coping with this hydration, there hasbeen adopted a coupling treatment using such a coupling agent as3-aminopropyltriethoxysilane, phenyltrimethoxysilane or3-methacryloxypropyltrimethoxysilane.

However, even if this coupling treatment is carried out, magnesia powderhaving a sufficiently high hydration resistance cannot be obtained, andmagnesia powder which has been subjected to the coupling treatmentcannot be used as a filler or the like. Moreover, the coupling treatmentrequires complicated steps of filtration, drying, rough pulverizationand calcination, and the treatment is very expensive.

SUMMARY OF THE INVENTION

Under this background, we made research and as the result, it was foundthat when high-purity magnesium oxide powder is used as the inorganicpowder, the thermal conductivity and metal abrasion resistance and thereliability to the performance of an obtained electronic part are highlyimproved. We have now completed the present invention bases on thisfinding.

Furthermore, in order to overcome the above-mentioned difficulties, thepresent invention is to provide a magnesia powder having a highhydration resistance and a process for simply preparing this magnesiapowder having a high hydration resistance.

More specifically, in accordance with a first aspect of the presentinvention, there is provided a resin composition for sealing electronicparts, which comprises a thermosetting resin and, incorporated therein,high-purity magnesium oxide.

In accordance with a second aspect of the present invention, there isprovided a hydration-resistant magnesia powder consisting of particleshaving the surface covered with a continuous and uniform coating ofsilica.

In accordance with a third aspect of the present invention, thishydration-resistant magnesia powder is prepared according to a processcomprising introducing a heated vapor of an organic silicate compoundinto a reactor of a fluidized bed of magnesia powder heated at 100° to600° C. so that the concentration of the organic silicate compound is 1to 20 mole %, and precipitating a coating of silica on the surfaces ofmagnesia particles by thermal decomposition and/or hydrolysis of theorganic silicate compound on the surfaces of magnesia particles.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As the high-purity magnesium oxide valuably used in the presentinvention, there are preferably used monocrystalline magnesium oxide andmagnesium oxide in which the total content of uranium and thorium islower than 10 ppb.

Monocrystalline magnesium oxide prepared, for example, by heatingmetallic magnesium at a high temperature to vaporize magnesium andoxidizing the resulting gaseous magnesium with oxygen in the gas phaseis preferably used. Monocrystalline magnesium oxide powder preparedaccording to this method has a very low content of impurities such aschlorine ion and bromine ion and is excellent in the dispersibility whenkneaded with a resin. Moreover, this magnesium oxide powder ischaracteristic over silica in that the hardness is lower than thehardness of silica.

Furthermore, magnesium oxide can be prepared by sufficiently distillingmetallic magnesium preferably so that the total content of radioactiveelements such as uranium and thorium is reduced below 500 ppb, furtherevaporating the refined magnesium and oxidizing the highly purifiedmagnesium with oxygen in the gas phase. In the so-obtained magnesiumoxide, the total content of radioactive elements such as uranium andthorium is reduced below 10 ppb, preferably to the trace. When thismagnesium oxide is used as a fuller, since the content of alkali metalions such as Na⁺ and K⁺ and halogen ions such as Cl⁻, which causedegradation of the moisture resistance of the resin and corrosion of analuminum element, is very low, occurrence of these undesirable phenomenacan be prevented. Accordingly, this magnesium oxide is especiallypreferred as a filler for the resin composition for sealing electronicparts according to the present invention.

In order to improve the kneading property with the resin, the fillingproperty at the resin-sealing step and the thermal conductivity, it ispreferred that the particle size distribution of the magnesium oxide besuch that particles having a particle size larger than 40μ occupy lessthan 10% by weight of the total particles, particles having a particlesize of 1 to 40μ occupy 80 to 90% by weight of the total particles andparticles having a particle size smaller than 1μ occupy less than 10% byweight of the total particles.

The resin composition for sealing electronic parts according to thepresent invention can be prepared by incorporating the above-mentionedmagnesium oxide into a thermosetting resin, adding a curing agent andother additives to the mixture and uniformly blending or kneading themixture by ordinary mixing and kneading means, such as melt-kneading ina vessel, melt-kneading by a roll or melt-kneading by an extruder.

As the mixing ratio of the magnesium oxide is increased, the thermalconductivity is proportionally increased but the flowability of thecomposition is reduced. If the amount of the magnesium oxide is toolarge, an electronic part cannot be completely sealed and many unfilledvoids are formed, and there is a risk of reduction of the moistureresistance. Furthermore, when the resin is filled under a pressure,there is a risk of breaking of a weak portion of an electronic part, forexample, a bonding wire connecting a semiconductor element to a leadportion. On the other hand, if the amount of the magnesium oxide is toosmall, the effect is too low and the intended object of the presentinvention cannot be attained. Therefore, it is preferred that the amountof the magnesium oxide be 10 to 85% by weight based on the totalcomposition.

In the present invention, the above-mentioned magnesium oxide alone canbe used as the filler, but an inorganic filler other than the magnesiumoxide, for example, silica such as crystalline silica or fused silica,alumina, calcium carbonate or talc, can be used in combination with themagnesium oxide. When this inorganic filler is used, it is preferredthat the total amount of the magnesium oxide and the inorganic filler be15 to 85% by weight based on the total composition. If the total amountof the magnesium oxide and the inorganic filler is smaller than 15% byweight based on the total composition, the thermal expansion coefficientor the mechanical strength is insufficient and no good results can beobtained.

In the present invention, as the thermosetting resin, there can be useda variety of known thermosetting resins such as phenolic resins,silicone resins, epoxy resins and polyimide resins, and epoxy resinssuch as a cresol-novolak epoxy resin, a phenol-novolak epoxy resin, abisphenol A type epoxy resin, a brominated phenol-novolak epoxy resinand an alicyclic epoxy resin are preferred. As the curing agent for thethermosetting resin, known curing agents can be used in the presentinvention. For example, there can be mentioned amine type curing agentssuch as diaminodiphenylmethane, diaminodiphenyl-sulfone andmetaphenylene-diamine, acid anhydride type curing agents such asphthalic anhydride, pyromellitic anhydride and maleic anhyride, andnovolak resin type curing agents such as a phenol-novolak resin and acresol-novolak resin. A curing promoter such as imidazole, an imidazolederivative, a tertiary amine derivative or a phosphine derivative may beadded. Moreover, a parting agent, a coupling agent, a flame retardantand the like may be added according to need.

In the resin composition for sealing electronic parts according to thepresent invention, α-rays causing an error in a semiconductor elementare not emitted, uranium or thorium is not substantially contained, theheat-dissipating property is excellent and the thermal conductivity isvery high. Therefore, an industrially very valuable sealant can beprovided according to the present invention.

The resin composition of the present invention is molded into anoptional shape after incorporation of the respective components. Sealingof an electronic part with the resin composition of the presentinvention can be accomplished by known means such as cast molding,compression molding, transfer molding or injection molding.

It is particularly preferred that magnesium oxide to be used as a fillerfor the above-mentioned resin composition for sealing electronic partsbe resistant to hydration. Therefore, in accordance with the presentinvention, there also are provided a hydration-resistant magnesia powderand a process for the preparation thereof.

In the present invention, various organic silicate compounds such astetraethoxysilane, methyltriethoxysilane and ethyltriethoxysilane can beused as the starting organic silicate.

By advancing chemical adsorption of radical complexes formed bydecomposition of the organic silicate compound on active sites on thesurfaces of magnesia particles and/or condensation reaction between theOH groups left on the surfaces of the particles and the organic silicatecompound, an organic substance-containing silica polymer is precipitatedon the particles of the magnesia powder. By further thermaldecomposition and/or hydrolysis of this organic substance-containingsilica polymer, a dense and uniform coating of silica is precipitated onthe particles of the magnesia powder. The thickness of this coating canbe optionally controlled according to the amount of the organic silicatecompound supplied into a fluidized bed reaction vessel, that is, theconcentration of the gas of the organic silicate compound, and/or thereaction time.

The temperature of the fluidized bed reaction vessel is maintained at100° to 600° C., preferably 350° to 450° C. If the temperature of thefluidized bed reaction vessel is lower than 100° C., the decompositionof the organic silicate compound is insufficient and a dense coating ofsilica cannot be formed on the magnesia particles. If the temperature ofthe fluidized bed reaction vessel is higher than 600° C., since theorganic silicate compound is decomposed in the gas phase, silica powderis precipitated in the free state and the magnesia particles are notcovered with silica in a good state. Moreover, if the temperature of thefluidized bed reaction vessel is lower than 100° C., physical adsorptionof the organic silicate compound to the inner wall of the reactionvessel is caused, and if the temperature of the fluidized bed reactionvessel is higher than 600° C., a thin film of silica is precipitated onthe inner wall of the reaction vessel by the chemical adsorption, andthe thin film is peeled and incorporated into the magnesia powder or thepeeled film disturbs the fluidization.

The concentration of the organic silicate compound in the gas suppliedinto the fluidized bed reaction vessel is 1 to 20 mole %, preferably 4to 8 mole %. If the concentration is lower than 1 mole %, since thepartial pressure of the organic silicate compound in the gas is too low,silica is hardly precipitated on the particles of the magnesia powder.If the concentration is higher than 20 mole %, the ratio of silicadeposited on the particles of the magnesia powder to the total amount ofthe supplied organic silicate compound is lower than 10%, and the yieldis drastically reduced and the process becomes economicallydisadvantageous. If the concentration of the organic silicate compoundin the supplied gas is adjusted to 4 to 8 mole %, the above-mentionedratio can be increased to 95% or higher.

If steam is simultaneously supplied in an amount of 0.1 to 20.0 molesper mole of the organic silicate compound supplied to the fluidized bedreaction vessel, the decomposition of the organic silicate compound ispromoted and the ratio of the amount of silica deposited on theparticles of the magnesia powder to the total amount of the suppliedorganic silicate compound can be increased to 99% or higher.

If the above-mentioned conditions of the present invention are adopted,by thermal decomposition and/or hydrolysis of the organic silicatecompound, silica is selectively precipitated on the particles of themagnesia powder but is hardly precipitated on the inner wall of thereaction vessel. The reason is that the coating of silica isprecipitated by the mutual action of the organic silicate compound withthe active sites on the surfaces of the particles of the magnesia powderand/or the OH groups left on the surfaces of the particles of themagnesia powder, as pointed out hereinbefore.

Since complicated steps are not necessary for the process for thepreparation of hydration-resistant magnesia powder according to thepresent invention, the preparation can be conducted in a discontinuousmanner or a continuous manner.

Moreover, since an organic silicate compound is used as the startingmaterial, the reaction temperature is relatively low and no corrosivegas is generated. Accordingly, hydration-resistant magnesia powder canbe industrially advantageously prepared according to the presentinvention.

In accordance with the present invention, by incorporating high-puritymagnesium oxide in a thermosetting resin, there can be provided anexcellent resin composition for sealing electronic parts, which isdistinguishable over the conventional compositions in that occurrence ofan error in a memory element by radiations is prevented and the heatdissipating property is very good.

The hydration-resistant powder of the present invention is highlyexcellent in the hydration resistance and it can be used as a filler invarious fields without hydration.

The hydration-resistant magnesia powder-preparing process of the presentinvention is advantageous over the conventional coupling treatmentprocess in that complicated steps such as filtration, drying, roughpulverization and calcination are not necessary and magnesia powderhaving a high hydration resistance can be prepared at a low cost.

Furthermore, hydration-resistant magnesia powder having an averageparticle size within a broad range of 0.01 μm to scores of μm and atotal content of uranium and thorium lower than 1 ppb can be provided.

The present invention will now be described in detail with reference tothe following examples that by no means limit the scope of theinvention.

REFERENTIAL EXAMPLE 1

This example illustrates the preparation of monocrystalline magnesiumoxide.

An oxidation reaction vessel was charged with magnesium having a purityof 99.9%, which was gasified by heating at 900° C., and argon gas havinga purity of 99.9% as a diluent so that the vapor pressure of magnesiumwas 0.04 atmosphere. Oxidation was carried out at 1000° C. whileintroducing oxygen gas having a purity of 99.9% into the reactionvessel, whereby monocrystalline magnesium oxide having a purity of99.9%, in which the particle size was mainly within a range of 0.01 to20 μ, was obtained.

REFERENTIAL EXAMPLE 2

This example illustrates the preparation of magnesium oxide.

High-purity magnesium (in which the total content of uranium and thoriumwas 490 ppb) obtained by distillation purification of metallic magnesiumwas gasified by heating at 1150° C. and introduced into an oxidationreaction vessel, and argon gas having a purity of 99.9% was introducedas a diluent into the reaction vessel so that the vapor pressure ofmagnesium was 0.04 atmosphere. Oxidation was carried out at 1000° C.while introducing oxygen gas having a purity of 99.9% into the reactionvessel. During the oxidation, the formed magnesia fine particles werecirculated and fusion growth was effected in an oxidative flame toobtain magnesium oxide having a purity of 99.99%, in which the totalcontent of uranium and thorium was 0.7 ppb.

The particle size distribution of the obtained magnesium oxide was asfollows.

Particles having a particle size smaller than 1 μ: 8% by weight

Particles having a particle size of 1 to 40 μ: 86% by weight

Particles having a particle size larger than 40 μ: 6% by weight

EXAMPLE 1

A mixture comprising 198 parts by weight of an o-cresol-novolak epoxyresin (having an epoxy equivalent of 214 and a softening point of 84°C.), 22 parts by weight of a brominated phenol-novolak epoxy resin(having an epoxy equivalent of 275 and a softening point of 84° C.), 110parts by weight of a phenolic resin (having a softening point of 80°C.), 33 parts by weight of antimony trioxide, 826 parts by weight of themonocrystalline magnesium oxide obtained in Referential Example 1, 4parts by weight of 2-ethyl-4-methylimidazole, 4 parts by weight ofcarnauba wax and 4 parts by weight of a silane coupling agent (A-187supplied by Nippon Unicar) was melt-kneaded at 80° to 100° C. by atwo-roll mixer and was then pulverized to obtain a resin composition forsealing electronic parts.

EXAMPLE 2

A molding material was prepared in the same manner as described inExample 1 except that instead of 826 parts by weight of themonocrystalline magnesium oxide used in Example 1, there were used 413parts by weight of the same monocrystalline magnesium oxide and 413parts by weight of fused silica (RD-8 supplied by Tatsumori-sha).

EXAMPLE 3

A molding material was prepared in the same manner as described inExample 1 except that instead of 826 parts by weight of themonocrystalline magnesium oxide used in Example 1, there was used 826parts by weight of the magnesium oxide obtained in Referential Example2.

EXAMPLE 4

A molding material was prepared in the same manner as described inExample 3 except that 600 parts by weight of the magnesium oxide wasused instead of 826 parts by weight of the magnesium oxide.

COMPARATIVE EXAMPLE 1

A molding material was prepared in the same manner as described inExample 1 or 3 except that 826 parts by weight of fused silica obtainedby fusing natural siliceous stone was used instead of the magnesiumoxide used in Example 1 or 3.

COMPARATIVE EXAMPLE 2

A molding material was prepared in the same manner as described inExample 1 or 3 except that 826 parts by weight of magnesium oxideobtained by pulverizing sea water magnesia clinker, which had a totaluranium and thorium content of 210 ppb and had such a particle sizedistribution that particles having a size larger than 40μ occupied 2% byweight of the total particles, particles having a size of 1 to 40μoccupied 50% by weight of the total particles and particles having asize smaller than 1 μ occupied 48% by weight of the total particles, wasused instead of 826 parts by weight of the magnesium oxide used inExample 1 or 3.

The characteristics of the so-obtained resin compositions for sealingelectronic parts were evaluated.

More specifically, the resin compositions obtained in Examples 1 through4 and Comparative Examples 1 and 2 were molded according to the transfermolding method and the characteristics of the molded products wereevaluated. The obtained results are shown in Tables 1 and 2.

It is seen that the molded products obtained in Examples 1 and 2 arecharacteristic over the molded product obtained in Comparative Example 1in the point that the abrasion loss of the metal by rubbing with metalis small and the molded article has a high thermal conductivity. It alsois seen that the molded articles obtained in Examples 1 and 2 aresatisfactory in electric and mechanical characteristics.

Furthermore, it is seen that the molded articles obtained in Examples 3and 4 are characteristic over the molded articles obtained inComparative Examples 1 and 2 in that the total uranium and thoriumcontent is extremely low, the α-ray intensity is low and the thermalconductivity is high. It also is seen that the molded articles obtainedin Examples 3 and 4 are satisfactory in electric and mechanicalcharacteristics.

                  TABLE 1    ______________________________________                                  Comparative                Example 1                        Example 2 Example 1    ______________________________________    Thermal Conductivity                  45        29        14    (10.sup.-4 cal/c · sec · °C.)    Flexural Strength                  12.1      12.8      13.0    (Kg/mm.sup.2)    Volume Resistivity                  4.8 × 10.sup.17                            6.0 × 10.sup.15                                      8.0 × 10.sup.15    (Ω-cm)    Abrasion Metal Amount*                  0.02      0.04      0.06    (mg)    ______________________________________     *the abrasion amount of the metal when the molded article was rubbed with     a metal sheet having a contact area of 5 cm.sup.2 10.sup.4 times

                  TABLE 2    ______________________________________                               Com-     Com-                               parative parative               Example                      Example  Example  Example               3      4        1        2    ______________________________________    Uranium and Thorium                 0.5      0.4      65     150    Content (ppb)    α-Ray Intensity                 below    below    0.06   0.14    (α · cm.sup.-2 · h.sup.-1)                 0.0001   0.0001    Thermal Conductivity                 45       37       14     36    (10.sup.-4 cal/cm · sec ·    °C.)    Flexural Strength                 12.1     11.5     13.0   11.8    (Kg/mm.sup.2)    Volume Resistivity                 4.2 ×                          3.0 ×                                   8.0 ×                                          8.2 ×    (Ω-cm) 10.sup. 17                          10.sup.16                                   10.sup.15                                          10.sup. 16    ______________________________________

EXAMPLE 5

In a fluidized bed reaction vessel heated at 400° C., 100 g of magnesiapowder prepared by gas phase oxidation of heated vapor of metallicmagnesium and having an average particle size of 0.055 μm was fluidized,and heated vapor of tetraethoxysilane was supplied into the reactionvessel together with entrained air and reaction was conducted for 1hour. The tetraethoxysilane concentration in the feed gas was 7 mole %and the total gas flow rate was 3.7 l/min. When the molar ratio ofsilica deposited on the magnesia particles SiO₂ /(MgO+SiO₂) molar ratiowas determined, it was found that this molar ratio was 13.9 mole %. Ifthe thickness of the silica coating was calculated from this amount, itwas found that the thickness was 3.5 nm.

The so-prepared hydration-resistant magnesia powder was dispersed inconcentrated hydrochloric acid and allowed to stand in this state forone week, whereby only the interior magnesia particles were dissolvedout. When the residual silica was observed by an electron microscope,hollow shell-like particles were found. Thus, it was confirmed that theparticles of the magnesia powder were covered with a very uniformcoating of silica.

The hydration resistance test of the obtained hydration-resistantmagnesia powder was carried out in the following manner. In 200 cc ofdistilled water was dispersed 5 g of the powder, and the dispersion wasstirred at 25° C. for 5 or 72 hours or at 100° C. for 24 hours, and thedispersion was filtered and the recovered solid was dried at 105° C. for5 hours. With respect to the obtained powder, the ignition loss wasmeasured according to JIS R-5202 and the identification was carried outby an X-ray diffractometer (Model MINI FLEX D-3F supplied by RigakuDenki). The obtained results are shown in Table 3.

EXAMPLE 6

Hydration-resistant magnesia powder was prepared in the same manner asdescribed in Example 5 except that the average particle size of themagnesia powder before coating with silica was 0.016 μm. The amount ofsilica precipitated on the magnesia particles was 18.1 mole %, and thethickness of the silica coating calculated from this amount was 1.3 nm.

In the same manner as described in Example 5, it was confirmed that themagnesia particles were covered with a very uniform silica coating. Eventhough very fine magnesia powder having an average particle size of0.016 μm were used as the starting material, the particles were notcovered in the aggregated state, and the obtained powder had a very highdispersion degree.

The hydration resistance test of the obtained powder was carried out inthe same manner as described in Example 5. The obtained results areshown in Table 3.

EXAMPLE 7

Hydration-resistant magnesia powder was prepared in the same manner asdescribed in Example 5 except that the temperature of the fluidized bedreaction vessel was changed to 200° C. The amount of silica precipitatedon the magnesia particles was 2.3 mole %, and the thickness of thesilica coating calculated from this amount was 0.6 nm.

The hydration resistance test of the obtained powder was carried out inthe same manner as described in Example 5. The obtained results areshown in Table 3.

EXAMPLE 8

Hydration-resistant magnesia powder was prepared in the same manner asdescribed in Example 5 except that steam in an amount equimolar totetraethoxysilane was simultaneously supplied into the fluidized bedreaction vessel. The amount of silica deposited on the magnesiaparticles was 18.5 mole %. Although the amount of tetraethoxysilane wasequal to that in Example 5, the amount of silica deposited on themagnesia particles was increased in this example.

The hydration resistance test of the powder was carried out in the samemanner as described in Example 5. The obtained results are shown inTable 3.

EXAMPLE 9

Hydration-resistant magnesia powder was prepared in the same manner asdescribed in Example 5 by using granular magnesia having a total uraniumand thorium content of 0.8 ppb and an average particle size of 18.2 μm,which was obtained by circulating magnesia fine particles, formed by gasphase oxidation of a heated vapor of metallic magnesium, in an oxidativeflame to effect fusion growth. The reaction time was changed to 10minutes. The amount of silica deposited on the magnesia particles was1.9 mole %, and the thickness of the silica coating calculated from thisamount was 0.2 μm. The hydration resistance test of the so-obtainedhydration-resistant magnesia powder was carried out in the same manneras described in Example 5. The obtained results are shown in Table 3.

Incidentally, the total uranium and thorium content of the usedtetraethoxysilane was lower than 0.05 ppb, and the tetraethoxysilane washighly pure. Accordingly, the obtained hydration-resistant magnesia finepowder had a total uranium and thorium content of 0.8 ppb. Therefore,when this powder was used as a semiconductor sealing material, an errorby radiations from uranium and thorium could be avoided.

COMPARATIVE EXAMPLE 3

Hydration-resistant magnesia powder was prepared in the same manner asdescribed in Example 5 except that the temperature of the fluidized bedreaction vessel was changed to 90° C. The amount of silica deposited onthe magnesia particles was 0.1 mole %, and a continuous and uniformcoating was not precipitated on the magnesia particles.

The hydration resistance test of the powder was carried out in the samemanner as described in Example 5. The obtained results are shown inTable 3.

COMPARATIVE EXAMPLE 4

Hydration-resistant magnesia powder was prepared in the same manner asdescribed in Example 5 except that the tetraethoxysilane concentrationin the gas supplied to the fluidized bed reaction vessel was changed to0.5 mole %. The amount of silica deposited on the magnesia particles wassmaller than 0.1 mole %, and a continuous and uniform coating of silicawas not precipitated on the magnesia particles.

The hydration resistance test of the powder was carried out in the samemanner as described in Example 5. The obtained results are shown inTable 3.

COMPARATIVE EXAMPLE 5

Hydration-resistant magnesia powder was prepared in the same manner asdescribed in Example 5 except that the tetraethoxysilane concentrationin the gas supplied into the fluidized bed reaction vessel was changedto 22 mole %. The amount of silica deposited on the magnesia particleswas 13.5 mole % and almost equal to the amount of deposited silica inExample 5, though the amount of supplied tetraethoxysilane was more than3 times the amount of tetraethoxysilane supplied in Example 5.Accordingly, the process was economically disadvantageous.

COMPARATIVE EXAMPLE 6

The hydration resistance test of magnesia powder having an averageparticle size of 0.055 μm, which was obtained by gasphase oxidation of avapor of metallic magnesium, was carried out in the same manner asdescribed in Example 5. The obtained results are shown in Table 3.

COMPARATIVE EXAMPLE 7

The hydration resistance test of magnesia powder having an averageparticle size of 0.016 μm, which was obtained by gas phase oxidation ofa vapor of metallic magnesium, was carried out in the same manner asdescribed in Example 5. The obtained results are shown in Table 3.

COMPARATIVE EXAMPLES 8 THROUGH 13

The magnesia powder used in Referential Example 3 was subjected to thecoupling treatment with 3-aminopropyl-triethoxysilane (ComparativeExample 8), phenylmethoxysilane (Comparative Example 9),3-methacryloxypropyltrimethoxysilane (Comparative Example 10),methyltrimethoxysilane (Comparative Example 11), silicone oil(Comparative Example 12) or hexamethyldisilazane (Comparative Example13) according to the known method.

With respect to each of the treated powders, the hydration resistancetest was carried out in the same manner as described in Example 5. Theobtained results are shown in Table 3.

                                      TABLE 3    __________________________________________________________________________           After 5 Hours' Stirring                         After 72 Hours' Stirring                                       After 24 Hours' Stirring           in Water at 25° C.                         in Water at 25° C.                                       in Hot Water at 100° C.           Ignition                X-Ray Diffrac-                         Ignition                              X-Ray Diffrac-                                       Ignition                                            X-Ray Diffrac-           Loss tometry* Loss tometry* Loss tometry*           (%)  MgO                   Mg(OH).sub.2                         (%)  MgO                                 Mg(OH).sub.2                                       (%)  MgO                                               Mg(OH).sub.2    __________________________________________________________________________    Example 5           1.4  ⊚                   x     1.8  ⊚                                 x     2.0  ⊚                                               x    Example 6           2.5  ⊚                   x     2.9  ⊚                                 x     3.0  ⊚                                               x    Example 7           1.8  ⊚                   x     2.1  ⊚                                 x     10.9 ⊚                                               Δ    Example 8           1.1  ⊚                   x     1.5  ⊚                                 x     1.5  ⊚                                               x    Example 9           0.1  ⊚                   x     0.1  ⊚                                 x     0.1  ⊚                                               x    Comparative           30.1 x  ⊚                         30.8 x  ⊚                                       32.2 x  ⊚    Example 3    Comparative           31.5 x  ⊚                         32.0 x  ⊚                                       32.1 x  ⊚    Example 4    Comparative           32.8 Δ                   ⊚                         33.0 x  ⊚                                       33.0 x  ⊚    Example 6    Comparative           35.1 x  ⊚                         35.3 x  ⊚                                       35.2 x  ⊚    Example 7    Comparative           27.1 Δ                   ⊚                         30.8 x  ⊚                                       32.2 x  ⊚    Example 8    Comparative           26.8 Δ                   ⊚                         31.6 x  ⊚                                       33.8 x  ⊚    Example 9    Comparative           28.8 Δ                   ⊚                         32.8 x  ⊚                                       31.5 x  ⊚    Example 10    Comparative           29.5 Δ                   ⊚                         34.1 x  ⊚                                       32.1 x  ⊚    Example 11    Comparative           25.5 Δ                   ⊚                         30.0 Δ                                 ⊚                                       34.1 x  ⊚    Example 12    Comparative           20.1 Δ                   ⊚                         29.6 Δ                                 ⊚                                       33.0 x  ⊚    Example 13    __________________________________________________________________________     Note     In the results of the measurement of the Xray diffractometry, mark "     ⊚ " indicates the main component, mark "Δ" indicates     the minute amount component, and mark "x" indicates no detection.

In the foregoing examples, tetraethoxysilane was used, but when organicsilicate compounds other than tetraethoxysilane were used, the reactioncould be similarly carried out, and similar results were obtained.However, in case of some organic silicate compounds, if oxygen ispresent in an atmosphere of the reaction vessel, decomposition ispromoted by oxidation and silica powder alone is precipitated in the gasphase. Accordingly, in this case, the atmosphere of nitrogen or argonshould be adopted.

REFERENTIAL EXAMPLE 3

100 g of magnesium oxide powder prepared by the same procedure as inReferential Example 2 was fluidized in a fluidized bed reaction vesselheated at 400° C., and heated vapor of tetraethoxysilane was suppliedinto the reaction vessel together with oxygen gas of a purity of 99.9%and nitrogen gas of a purity of 99.9% over the course of 10 minutes toobtain hydration-resistant magnesia powder. The proportions of therespective tetraethoxysilane, oxygen and nitrogen gases were 7%, 19% and74% by volume. The tetraethoxysilane used above was prepared byredistillation and had a purity of 99.9% and a total U and Th content ofnot more than 0.05 ppb. When the amount of silica deposited on theobtained hydration-resistant magnesia particles was determined, it wasproved that the molar ratio of SiO₂ /(MgO+SiO₂) was 13.9 mole %. Fromthis amount, a thickness of the silica coating of 0.2 nm was calculated.The total content of U and Th was 0.7 ppb, which proved that U and Thwere not incorporated into the magnesia powder in the fluidized bedreaction vessel.

EXAMPLE 10

The procedure of Example 1 or 3 was repeated except that the magnesiumoxide was replaced by the same amount of the hydration-resistantmagnesia powder obtained in Referential Example 3.

EXAMPLE 11

A molding material was prepared in the same manner as described inExample 10 except that the hydration-resistant magnesia powder was usedin an amount of 600 parts by weight instead of 826 parts by weight.

                                      TABLE 4    __________________________________________________________________________                Before Autoclave                                After Autoclave                Treatment       Treatment                        Comparative     Comparative                Example Example Example Example                10  11  1   2   10  11  1   2    __________________________________________________________________________    Uranium and 0.5 0.4 65  150 0.5 0.4 65  150    Thorium Content    (ppb)    α-Ray Intensity                below                    below                        0.06                            0.14                                below                                    below                                        0.06                                            0.14    (α · cm.sup. -2 · h.sup.-1)                0.0001                    0.0001      0.0001                                    0.0001    Thermal Conductivity                44  36  14  36  43  36  12  18    (10.sup.-4 cal/cm · sec · °C.)    Flexural Strength                12.1                    11.5                        13.1                            11.8                                12.0                                    11.5                                        13.0                                            8.1    (kg/mm.sup.2)    Volume Resistivity                3.9 ×                    3.0 ×                        8.0 ×                            8.2 ×                                3.8 ×                                    2.9 ×                                        7.7 ×                                            5.5 ×    (Ω° cm)                10.sup.17                    10.sup.16                        10.sup.15                            10.sup.16                                10.sup.17                                    10.sup.16                                        10.sup.15                                            10.sup.13    __________________________________________________________________________

We claim:
 1. A hydration-resistant magnesia powder consisting ofparticles of magnesia having the surface covered with a continuous anduniform coating of silica.
 2. A process for the preparation of ahydration-resistant magnesia powder, which comprises introducing aheated vapor of an organic silicate compound into a reactor of afluidized bed of magnesia powder heated at 100° to 600° C. so that theconcentration of the organic silicate compound is 1 to 20 mole %, andprecipitating a coating of silica on the surfaces of magnesia particlesby thermal decomposition and/or hydrolysis of the organic silicatecompound on the surfaces of magnesia particles.
 3. A process accordingto claim 2, wherein the temperature of the fluidized bed reactor is 350°to 450° C.
 4. A process according to claim 2, wherein the concentrationof the heated vapor of the organic silicate compound is 4 to 8 mole %.5. A process according to claim 2, wherein steam is supplied togetherwith the vapor of the organic silicate compound.
 6. A process accordingto claim 5, wherein steam is supplied in an amount of 0.1 to 20.0 molesper mole of the organic silicate compound.